The ability of cells to proliferate independently of a surface substratum is a property that distinguishes transformed cells from normal cells. Current thinking is that this ability of tumor cells is brought about by the production in tumor cells of anchorage-independent growth conferring factors, or transforming growth factors (TGF). New observations that we and others have made indicate that normal tissues in addition to tumor tissues can make TGF. For example, we have found that TGF activities are made by or accumulate in proliferating bovine mammary gland, in rat, mouse and human adenocarcinomas and in fact are present in large amounts in human milk. The TGF activities in human milk and human mammary tumors have been partially purified. The major species from the two sources have identical PI's and are probably the same protein. TGF activities have also been detected in several clones of MCF-7 and Clouser, human tumor cell lines, grown in nude mice. These findings indicate that tumor cells need not necessarily make their own TGF to express the anchorage-independent growth phenotype but rather upon transformation they can respond to factors normally made by, and probably physiologically important to the host. Consistent with this concept is our finding that some rodent mammary carcinomas and 1 human mammary tumor contained no detectable TGF activity.